The depression or decrease in the freezing point of the solvent (Cryoscopy)

The Freezing point is the temperature at which liquid and solid states of the substance are in equilibrium with each other or it can be defined as the temperature at which the liquid and the solid states of the substance have the same vapour pressure. It is observed that freezing point of the solution is always less than the freezing point of pure solvent. Therefore the freezing point of sea water is low than that of pure water. The depression in freezing point ΔT or  ΔT_f of a solvent is the difference in the freezing point of the pure solvent (T_s) and the solution (T_sol).

                    ΔT_{s -} ΔT_sol = Δ T_f or ΔT

NaCl or CaCl₂ (anhydrous) are used to clear the snow on the roads. They depress the freezing point of the water and hence reduce the temperature of the formation of the ice.

Depression in the freezing point is determined by the Beckmann's method and the Rast's camphor method. Study of depression in the freezing point of a liquid in which a non-volatile solute is dissolved in it is termed as cryoscopy.

Important relations concerning depression in freezing point.

(1) Depression in freezing point is directly proportional to the lowering of vapour pressure.  

(2) 

Where K_f = molal depression constant or cryoscopic constant; m = Molality of the solution (i.e., no. of moles of solute per 1000g of the solvent);   ΔT_f = Depression in freezing point

(3)   

where K_f is molal depression constant and defined as the depression in freezing point produced when 1 mole of the solute is dissolved in 1 kg of the solvent. w and W are the weights of solute and solvent and m is the molecular weight of the solute.

(4) 

where T_{0 =} Normal freezing point of the solvent; L_f Latent heat of fusion/g of solvent; K_f for water is 1.86 deg-kg mol^-1

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